1. Field of the Invention
The present invention relates to an ultrasonic transducer apparatus for breaking a calculus or eliminating a tumor utilizing ultrasonic oscillations.
2. Description of the Related Art
As shown in FIG. 1, a conventional ultrasonic transducer apparatus of this type includes a driving unit 1 having a driving circuit 2, and an ultrasonic transducer probe 3 which has an ultrasonic transducer 4 and which is detachable from the driving unit 1.
As shown in FIG. 2, an equivalent circuit of the ultrasonic transducer 4 includes an LCR series resonance circuit formed of an inductor 5, a capacitor 6, and a resistor 7, and a dumping capacitor Cd connected in parallel with the LCR series resonance circuit. When a voltage is applied to the ultrasonic transducer 4, currents il and id are supplied through the LCR series resonance circuit and the dumping capacitor Cd, respectively. Of the currents il and id, only the current il is converted into ultrasonic oscillations. Therefore, it is most efficient to drive the ultrasonic transducer 4 at a resonance frequency of the LCR series resonance circuit. The resonance frequency of the series resonance circuit is referred to as a mechanical resonance frequency ft for the ultrasonic transducer 4 hereinafter.
Since a conductance G of the ultrasonic transducer 4 is maximum at the mechanical resonance frequency ft, the frequency ft is a rightmost point in a graph of an admittance Y (=G+jB) of the ultrasonic transducer 4, as shown in FIG. 3. FIG. 3 shows a locus of the admittance Y obtained when an angular frequency .omega. is a variable. Reference symbol B denotes a susceptance; and .omega., a driving angular frequency (=2.pi.f).
The conventional driving circuit 2 includes a phase-locked loop (PLL) circuit to lock the driving frequency when the conductance is maximum. The PLL circuit controls the driving frequency to make a phase difference between a voltage applied to the ultrasonic transducer 4 and a current supplied to the ultrasonic transducer 4, i.e., a susceptance, zero. As shown in FIG. 3, however, the center of an admittance characteristic circle of the ultrasonic transducer 4 is shifted in the positive direction of the susceptance by a capacitive susceptance .omega.Cd of the dumping capacitor Cd. Therefore, a lock point (point at which a susceptance is zero) PR obtained by the PLL does not coincide with the mechanical resonance point ft (point at which a conductance is maximum), i.e., the transducer 4 cannot be driven at the mechanical resonance frequency even if a phase difference between the voltage and the current is set to be zero. As a result, conversion efficiency of a driving signal into ultrasonic oscillations is poor.
An apparatus to eliminate the above drawback is disclosed in Published Unexamined Japanese Utility Model Application No. 54-136943. As shown in FIG. 4, in this apparatus, the driving unit 1 includes an inductor Ld arranged in parallel with the ultrasonic transducer 4 besides the driving circuit 2. According to this apparatus, as shown in FIG. 5, a capacitive susceptance .omega.Cd of the dumping capacitor Cd included in the ultrasonic transducer 4 can be canceled by an inductive susceptance (=1/.omega.Ld) of the inductor Ld. As a result, as shown in FIG. 5, the center of the admittance characteristic circle of the equivalent circuit of the ultrasonic transducer 4 is positioned on the axis where the susceptance is zero, and the lock point PR obtained by the PLL coincides with the mechanical resonance point ft. Therefore, the ultrasonic transducer 4 can be efficiently driven.
A capacitive susceptance of the dumping capacitor in the ultrasonic transducer probe is varied depending on its shape or the characteristics of the ultrasonic transducer included in the probe. For this reason, in the ultrasonic transducer apparatus in which different types of ultrasonic transducer probes can be connected to the above-mentioned driving unit shown in FIG. 4, capacitive susceptances of dumping capacitors in the ultrasonic transducers of all probes cannot be canceled. In other words, in a driving unit including only one inductor Ld, capacitive susceptances of different dumping capacitors cannot be perfectly canceled. Therefore, when various ultrasonic transducer probes are selectively connected to a driving unit as in an ultrasonic medical treatment apparatus in accordance with a target to be treated, various ultrasonic transducers cannot be driven at respective optimal mechanical resonance points. As a result, driving efficiency is poor.